In the Classroom edited by
View from My Classroom
David L. Byrum
Investigating the Cuprammonium Rayon Process in a High School Laboratory
Flowing Wells High School Tuscon, AZ 85716
Lauren J. Pickard and Mary E. Harris* John Burroughs School, St. Louis, MO 63124; *
[email protected] The chemical technology required to produce common products can serve as the basis for fascinating student research projects. We found this to be the case for the creation of rayon from cellulose. The cuprammonium rayon process provided challenges and surprises in a teacher–student joint investigation. The literature describes several different methods; we decided to find the “best” method for use in the high school laboratory. Our school offers the opportunity for independent study to any interested student and it is up to the student and teacher to set goals for the year of study. Credit is assigned on the basis of the number of meetings per week. The student (LJP) enrolled in an independent study course and we met twice a week for a semester to do experimental work. The project continued into a second semester for extensions to the rayon process and preparation of a science fair presentation. The choice of in-depth study of a particular chemical process rather than enrollment in a traditional second-year course required a one year commitment by both student and teacher, but it proved valuable to both of us. The Rayon Process Dissolving cellulose and then regenerating it in acid (or sometimes base) makes rayon. The procedure for making rayon involves aqueous ammonia or sodium hydroxide, which is reacted with aqueous copper(II) sulfate solution to precipitate copper(II) hydroxide. The solid is dissolved with concentrated aqueous ammonia to form Schweitzer’s reagent for the digestion of cellulose. Rayon is formed when cellulose is regenerated in an acid medium. Our initial laboratory time was spent trying the procedures in the literature, as there were several choices. It was puzzling as well as frustrating when dozens of unsuccessful trials were performed. Finally a successful method was identified. Then we concentrated on improving the formation of rayon fibers and later experimented with making rayon disks. We received advice about commercially prepared cuprammonium rayon through email correspondence with Courtaulds in the United Kingdom, the Kidney Center at Washington University in St. Louis, and Terumo Medical Corporation in New Jersey. Several Variations Were Identified In a literature search of the cuprammonium rayon process we found that several variations in starting materials are suggested. Summerlin and Ealy (1, 2), suggest adding drops of aqueous ammonia to a saturated copper(II) sulfate solution to form the copper(II) hydroxide precipitate. These authors advise the reader to discard the blue solution if it becomes too dark blue, which indicates the presence of tetraamminecopper(II) hydroxide. Shakhashiri (3) uses concentrated aqueous ammonia to make the copper(II) hydroxide precipitate and 0.5 M sulfuric acid to precipitate the rayon from the solution of cellulose in Schweitzer’s reagent. A Jour1512
nal article by Kauffman and Karbassi (4) reports use of 1 M sodium hydroxide instead of concentrated aqueous ammonia to make copper(II) hydroxide and use of 1 M copper(II) sulfate as a source of copper(II) ion. The Journal article by Washburn in 1928 (5) suggests that the concentration of aqueous ammonia used to form Schweitzer’s reagent does not affect the experiment. Knopp (6 ) uses copper(II) carbonate with concentrated aqueous ammonia to form the dark blue solution. The precipitate of copper(II) hydroxide does not need to be washed or filtered. The process we used most frequently is described in the article “Cuprammonium Rayon” in Chemical Demonstrations by Shakhashiri (3). This method calls for 12.5 g of copper(II) sulfate dissolved in 50 mL of distilled water to be precipitated in 6.5 mL of concentrated aqueous ammonia. The precipitate is then suction-filtered in a Buchner funnel and washed three times with cold water. To make Schweitzer’s reagent, add 75 mL of concentrated aqueous ammonia to the copper(II) hydroxide precipitate. The cellulose is regenerated in 2 M sulfuric acid. We Identified the Essential Features In the process of making copper(II) hydroxide precipitate, the addition of concentrated aqueous ammonia or sodium hydroxide is apparently not a critical step. We have found that one can shift the equilibrium to the complex ion, tetraamminecopper(II) hydroxide, by adding concentrated aqueous ammonia and then returning to the copper(II) hydroxide by adding saturated copper(II) sulfate solution. Cu(OH)2(s) + 4NH3(aq) Cu(NH3) 4(OH) 2(aq) Cu 2+(aq) + 2OH{(aq) + 4NH3(aq) Our best results occur when the pH of the solution in equilibrium with copper(II) hydroxide precipitate is between 4.5 and 6.4. It is not easy to tell what conditions give optimum production of the copper(II) hydroxide precipitate. As one approaches neutrality, excess Cu2+ will be in the discarded washings. For optimum results, the filtrate should be colorless, indicating that the reaction with the copper ions is complete. Once the precipitate is made and rinsed, the addition of concentrated aqueous ammonia is critical. We have not been able to do this by visual inspection of color. One needs a pH of 12.5 or higher to obtain Schweitzer’s reagent. We found that cellulose does not dissolve at pH 12.2: if more aqueous ammonia is added, the cellulose will quickly dissolve as the pH increases. The equilibrium reaction for cellulose dissolving in Schweitzer’s reagent (4 ) is nCu 2+ + (C6H10O5)n + 2nOH{
(CuC6H8O5)n + 2nH2O
α-Cellulose, by definition, is the alkali-resistant portion of cellulosic fibrous material (7). Schweitzer’s reagent is capable of dissolving α-cellulose, which is then regenerated to create
Journal of Chemical Education • Vol. 76 No. 11 November 1999 • JChemEd.chem.wisc.edu
In the Classroom
rayon. If cellulose is treated with 17.5% sodium hydroxide, the dissolved portion of the cellulose that can be precipitated by neutralization is called β -cellulose. γ -Cellulose is the portion that remains in solution after neutralization. α-Cellulose remains undissolved. Native cellulose consists almost entirely of αcellulose, which is used in pulp and paper production (8). Ahlstrom 54 and Whatman No. 1 filter papers consist almost entirely of α-cellulose. Other cellulose products such as roll cotton, sterile gauze pads, and Kleenex can be used as a source of cellulose for preparing rayon. Since cotton growers are now producing naturally colored cotton, it would be interesting to see if one could use these colored cotton fibers to make colored rayon. Making Rayon Disks Once the viscous solution of cellulose in Schweitzer’s reagent is ready for fiber formation, dilute sulfuric acid is used to precipitate the fiber. A pipet filled with the viscous solution delivers a stream of cellulose into the acid. It is advisable to use a small spatula to draw the fiber away from a pipet tip at the point of contact between the cuprammonium solution and a 2 M sulfuric acid solution. We wanted a larger sample of rayon, so we tried making disks by pouring the viscous solution into a clean dish and coating the bottom of the dish with a thin layer. We add the dilute sulfuric acid on top and swirl. The blue color will disappear and leave a translucent rayon disk. Pour off the acid, rinse with water and then allow the disk to dry in the air. Placing a 100-mL beaker with the mouth down on the disk will help hold the circular shape. The less viscous the solution, the clearer and more fragile the disk. The luster of the rayon is very evident when one makes a disk instead of a fiber. See Figure 1.
Reformulation through Alkali An older French patent by R. Linkmeyer (9) suggests using alkali solution to precipitate the cellulose. After the disk is poured in the Petri dish, add 40% sodium hydroxide to dehydrate the cellulose. Place the disk in a bath of tap water to remove most of the base. Once the base is washed off and 2 M sulfuric acid is added, the blue color leaves the disk as before. The rayon formed by this process seems to be stronger than when the acid reaction is used to regenerate cellulose. The patent claims that this process produces a more lustrous rayon, but the disks we prepared were not much different from others. Rayon Dialysis Membranes The cuprammonium rayon membranes used in kidney dialysis machines have a striking luster. We were able to obtain a free membrane unit from a local clinic and we sawed off the end to obtain some fibers. The rayon fibers can be dissolved and regenerated by the same procedure used to dissolve αcellulose in filter papers. Dialysis machines balance fluid intake and output for a patient who has kidney failure. The Terumo membrane dialyzer, shown in Figure 2, allows blood to flow through it in one direction while a dialysate (a fluid simulating blood) bathes the outside of the fibers as it flows in the opposite direction. In this way, the fibers trap urea and other wastes that build up in the blood. The cuprammonium fibers are hollow and have a pore diameter of 250 pm. The membranes may be formed either from cuprammonium rayon or from other synthetic materials (10). Plenty of Room for More Projects This project taught both student and teacher about use of the chemical literature and the Internet, acid–base and oxidation–reduction equilibria, industrial chemistry, and the history of chemistry. There is opportunity for more work on the making of rayon. Using different sources of cellulose as starting materials is one suggestion. Making stronger and longer fibers is another. Our rayon disks are fragile, but a method may be developed to strengthen them. Working on creating a more lustrous product would be another study. This was a rewarding experience for both of us and we learned and applied a lot of chemistry while we worked together. Literature Cited
Figure 1. Rayon disks from Ahlstrom filter paper.
Figure 2. Rayon disk from Terumo Hollow Fiber Dialyzer fibers.
1. Summerlin, L. R.; Ealy, J. I. Jr. Chemical Demonstrations: A Sourcebook for Teachers; American Chemical Society: Washington, DC, 1985; pp 128–129. 2. Summerlin, L. R.; Ealy, J. I. Jr. Chemical Demonstrations: A Sourcebook for Teachers, Vol. 1, 2nd ed.; American Chemical Society: Washington, DC, 1988; pp 174–175. 3. Dirreen, G. E.; Shakhashiri, B. Z. In Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 1; Shakhashiri, B. Z., ed.; University of Wisconsin Press: Madison WI, 1983; pp 247–249. 4. Kauffman, G. B.; Karbassi, M. J. Chem. Educ. 1985, 62, 878. 5. Washburn, E. R. J. Chem. Educ. 1928, 5, 96–98. 6. Knopp, M. A. J. Chem. Educ. 1997, 74, 401. 7. Kauffman, G. B. J. Chem. Educ. 1993, 70, 887–893. 8. Browning, B. L. Analysis of Paper; Dekker: New York, 1969; p 55. 9. Foltzer, J. Artificial Silk and Its Manufacture; Pitman and Sons: New York, 1926; p 140. 10. Binkley, L. S.; Hopkins, P. E.; Hudson, M. V.; Parker, J.; Wick, G. In Core Curriculum for theDialysis Technician; Module III, Principles of Dialysis; Amgen Inc. (Medical Media): New York, 1992; pp 1–14.
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